The invention relates to a drilling tool.
A drilling tool is known having a drill head which has a driven part which can be connected either to a drilling string or to a casing string, comprising a reversing valve associated with a drive motor for the drill head, by means of which operating apertures in the drive motor can be connected either to a pressure line or to a return line in order to set the driven part in motion in either clockwise or anticlockwise rotation, and comprising a control unit associated with the reversing valve which has a manually operated direction change over switch.
Drilling tools of this type are used on those sites where a fairly small number of drill holes have to be made and where the earth first has to be loosened using a drill string and raised to the surface, and then the earth is prevented from sliding back into the drill hole by sinking a casing pipe string. Whereas on large sites where a large number of these drill holes have to be made a special casing machine is used to drive the casing pipe string into the ground, on small sites the drill head is used to rotate and sink the casing pipe string. It goes without saying that in this process a mechanical connection must be made between the driven part of the drill head and the casing pipe string or drill string depending upon the status of the work. The sinking of the casing pipe string is also achieved by using the drill head to rotate it in order to reduce the tensile loads being exerted.
The drill holes thus made are generally filled with concrete mixed on site which, once set, forms a foundation pile. In order to improve the load carrying capacity of the site-mixed concrete, reinforcing cages are set into it. If, with the drill hole filled with site-mixed concrete and with a reinforcing cage inserted, the casing pipe string is now sunk, using the drill head working on the drill string to rotate it, then by setting the casing pipe string to rotate constantly in the same direction, the concrete mass, and thus also the reinforcing cage which is set into it is finally set in rotation. Uncontrolled positional changes of the reinforcing cage may occur as a result.
This problem does not arise with the drilling tools used on large sites because the casing machines used there exert a backward and forward rotary motion on the casing pipe string (producing a directional effect on the casing pipe string).
The invention provides a drilling tool with a drill head which has a driven part which can be connected to either a drilling string or to a casing string, the drilling tool comprising a reversing valve associated with a drive motor for the drill head, by means of which operating apertures in the drive motor can be connected wither to a pressure line or to a return line in order to set the driven part in motion in either clockwise or anticlockwise rotation, and comprising a control unit associates with the reversing valve which has a manually operated direction change-over switch, in which the control unit has an automatic direction change-over switch, the output signals from which can be used in the same way as those of the manual direction change-over switch to control the reversing valve, the manual direction change-over switch and the automatic direction change-over switch can, as an option, be activated by an operating mode selector, and the direction change-over frequency at which the direction change-over switch operates can be adjusted to the speed of the drive motor so that the driven part executes an osciallating rotary movement with limited angular stroke.
It is now recognised that even on the more simple drilling tools according to the invention, any uncontrolled movement of the reinforcing cages inserted into the drill hole during the rotation-assisted sinking of the casing pipe string can be reduced by providing an automatic direction change-over switch, the switching frequency of which is selected to allow for the speed of rotation of the driving mechanism operating on the casing pipe string when sinking the casing pipe so that the aforesaid driving mechanism only executes limited oscillating angular movements.
The driving forces exerted by these rapid oscillating movements on the concrete in-fill become accentuated in time, with the result that no net torque or net forces are exerted even on the reinforcing cage inserted into the concrete.
This advantage is obtained at little additional cost because the provision of an automatic direction change-over switch does not involve the need for any expensive hydraulic power system parts. If a stored programmable control system is used to control the direction reversing valve, then automatic direction change-over can simply be achieved by means of an additional program block.
Preferably, the angular stroke of the driven part is less than 90xc2x0 and is preferably between 20xc2x0 and 30xc2x0.
The selection of the direction change-over frequency which leads to the angular strokes of this embodiment on the one hand considerably aids the sinking of the casing pipe string by rotating it, whilst on the other hand no net driving of the concrete occurs.
The ejecting of drillings from the drill tool can be very easily automated on a drilling tool according to the invention by providing yet another additional small measure over and above the direction change-over switch already provided; namely an additional automatic direction change-over switch which operates at an ejection change-over frequency which in practice is generally lower than the direction change-over frequency. To achieve change-over by means of a stored programmable control necessitates only an additional program block.
Whereas the frequency of the ejection direction change-over switch is selected so that in operation the drill string connected to the driven part of the drill head accelerates to full speed and then is suddenly braked in order to eject the earth contained in the drill helix, the operating frequency of the direction change-over switch is adjusted so that, allowing for the speed of the driven part of the drill head, only the comparatively small backward and forward movement of the casing pipe string as described is obtained.
Preferably, a controllable throttle is inserted into the pressure line leading to the reversing valve or into the return line leading to the reversing valve, and the servomotor for which is excited as a function of the output signal from the operating mode selector.
This further embodiment of the invention produces the effect that when the drill head is used to produce a direction effect on a casing pipe string, the operating speed of the driven part is also automatically reduced. This is also an advantage for producing a gentle directional effect.
Preferably, the servomotor for the controllable throttle is a pressure medium motor which can be connected to a control pressure source via a fluid OR element either directly or with the insertion of a pressure reducer.
According to this embodiment of the invention, the speed change-over of the drill head can be achieved very accurately and reliably.
Preferably, one input of the OR element is connected directly and the other input is connected via the pressure reducer to the two connections on the outlet side of a 3/2 pilot valve, but preferably a 3/3 pilot valve, the input of which is connected to the control pressure source, and the pilot valve is brought into the first operating position by two output signals from the operating mode selector which, are provided to set normal drilling mode with manual direction reverals or automatic reversal at the ejection change-over frequency and is put into the second operating position by a third output signal from the operating mode selector which is provided to set automatic reversal at direction change-over frequency.
Preferably, the drilling tool comprises a master control switch which can both start and stop the operation of all the direction change-over switches.
This embodiment allows the operators to stop the drill head or set it in motion in the same way in all operating modes of the drill head.